Model Reactions Involving Ester Functional Groups during Thermo‐Oxidative Degradation of Biodiesel

Abstract

Biodiesel is a renewable fuel used in diesel engines that is typically blended with diesel fuel. However, biodiesel is susceptible to oxidation, which has the potential to produce higher molecular weight materials that may adversely impact vehicle fuel-system performance. To investigate the chemical reactions potentially important in biodiesel oxidation, four different types of chemical reactions involving esters were studied: (1) ester formation (reactions of acids with alcohols), (2) alcoholysis (reactions of alcohols with esters), (3) acidolysis (reaction of acids with esters), and (4) ester exchange (reactions between two esters). Experiments with representative model compounds were used to evaluate these reactions at 90 C with aeration; conditions previously used to simulate thermooxidative degradation during biodiesel aging. Reactions were monitored using gas chromatography, fourier transform infrared (FTIR) spectroscopy, and total acid number (TAN). Evidence is presented suggesting that alcoholysis and ester formation (Reactions 1 and 2), catalyzed by carboxylic acids, are important reactions of esters that could lead to larger molecules. Acidolysis (Reaction 3) proceeded at a comparatively slow rate and ester exchange reaction products (Reaction 4) were not detected under these aging conditions.

Thermal oxidation of model molecules to reveal vegetable oil polymerization studied by NMR spectroscopy and self‐diffusion

“…Thermal oxidation of plant oils and oleic acid concerns diverse areas of science such as food, health, nanomaterials, and renewable fuels. Conversion of oils to undesirable higher molecular weight species (polymerization) during frying or storage is detrimental to health (Patrikios & Mavromoustakos, 2014) and bio‐refinery processes (Ball et al, 2019; Barani, 2021). Polymer formation is a measure of edible oil deterioration (Paquette et al, 1985).…”

“…Recently evidence was presented for doubting ether cross‐links in thermally auto‐oxidized (no catalyst) vegetable oils. Model reactions between carboxylic acids and alcohols (90°C with aeration) showed facile ester formation (Ball et al, 2019). NMR spectroscopy found ester bond formation in thermally oxidized oleic acid (Bonetti & Parker Jr, 2019) and soybean oil (Hwang et al, 2020).…”